Currently there is considerable interest in the development of low-cost, reliable gyroscopes and researchers have proposed a variety of solutions to meet that challenge. Traditional military-grade gyroscope fabrication techniques are not scalable to high volume manufacturing or low production cost. The field of Micro Electro Mechanical Systems (MEMS) utilizes semiconductor fabrication techniques to construct microscopic mechanical systems, and hence provides the manufacturing model for low-cost inertial sensing systems. A variety of researchers have pursued MEMS gyroscope designs using a multiplicity of design and fabrication methods.
However, miniature gyroscopes have numerous technical obstacles related to the assembly and mass limitations of silicon-based elements. Some of the obstacles include small inertial masses, minute sense signals, and high volume packaging methods. To reduce electrical coupling, complicated circuit techniques have been developed to separate the drive and sense signals. To provide more angular momentum, researchers have developed specialized drive methods. To increase the sense signals, resonance matching and high Q oscillators effectively boost the system gain. However, it has previously been difficult to obtain many of the favorable operating parameters in a single device that is manufacturable in high volumes.
What is presented here is a silicon gyroscope that solves many of the inherent difficulties in the prior art to create a high performance device in a highly manufacturable fabrication sequence. The gyroscope requires no special materials or packaging in order to fabricate the device in high volumes. By simultaneous mechanical and electrical decoupling of the drive and sense oscillators, the requirements for complex signal processing and high gain oscillators are alleviated. Patented fabrication techniques impart unique features to the gyroscope device and enable a wide design window for tuning the gyroscope performance.
The gyroscope of the present invention differs from prior art in the manner in which it decouples, both electrically and mechanically, the drive and sense oscillators. The gyroscope of the present invention is also unique in that all the electrical drive and sense signals are integrated directly onto the released MEMS element, and not dependent upon substrate or buried electrodes in order to route electrical signals to the external package. This facilitates the electronics design, removes difficult processing and alignment steps, and ultimately allows industry standard packaging solutions to enable high volume manufacturing.
Micromachined gyroscopes commonly use oscillating rather than rotating members due to the obstacle that friction imposes at micron dimensions. Many gyroscopes rely on complex motion of a single resonating member to transduce angular rate. In so-called tuning fork designs, for example in U.S. Pat. No. 5,349,855 (Bernstein, et al) and U.S. Pat. No. 5,992,233 (Clark, et al), a micromachined device is resonated in plane and undergoes complex motion when subjected to gyroscopic torque. The angular rate is deconvolved using multiple electrode structures, complex vibrational modes, and clever signal processing. Nevertheless, the difficulty in these arrangements is the inherent mechanical coupling of the drive member with the sense member. In other words, the single mechanical oscillating structure results in high levels of electromechanical cross talk, manifested in high quadrature signals in the output electronics.
Substantial decoupling of the mechanical motions for drive and sense have been accomplished through the use of oscillating mechanical elements. In U.S. Pat. No. 5,555,765 (Greiff) and U.S. Pat. No. 5,955,668 (Hsu et. al), a single mechanical member is oscillated using rotationally symmetric drive electrodes. Subsequent gyroscopic motion transmits the Coriolis force into a second distinct rotational mode. In both patents, this sensing mode is transduced using buried electrodes beneath the micromechanical structure. Although markedly reducing the mechanical complexity, these devices require multi-level processing and typically limit the fabrication to thin-film materials, reducing overall sensitivity. Another implementation of decoupled mechanical designs is revealed by Geiger, et. al in xe2x80x9cNew Designs of Micromachined Vibrating Rate Gyroscopes with Decoupled Oscillation Modes.xe2x80x9d Again, the oscillating rotating member serves to energize the gyro but a buried electrode is required to transduce the output motion. All of these planar oscillating gyroscopes can be arranged into a number of configurations, each sensitive to gyroscopic input along a different axis. The orthogonal modes of rotationally oscillating gyroscopes are detailed in Cardarelli, et. al in U.S. Pat. No. 5,915,275.
None of the prior art, however, incorporate decoupled mechanical operation with the ability to electrically isolate the drive and sense signals on the same, released mechanical structure. Such a device is capable of substantially reducing cross coupling, both mechanical and electrical, improving the quality of the output signals and reducing the requirements for precision electrical transduction or difficult signal processing. It is the object of the present invention to provide such a device within the context of a highly manufacturable silicon MEMS process.
The invention is a micromachined planar oscillatory gyroscope with electrical and mechanical decoupling. Electrical decoupling relates to the ability to construct multiple electrically isolated regions within one mechanically connected structure. Mechanical decoupling refers to the physical mode separation of the drive and sense functions. Together these two forms of decoupling reduce electromechanical cross-talk, a major contributor to the zero rate output and zero rate output shift over temperature. In addition, the electrical decoupling greatly simplifies the sensing electronics.
The gyroscope of the invention is fabricated with a single-crystal silicon based fabrication technology. The dry etch process begins with a standard silicon wafer, out of which high-aspect ratio structures are sculpted. The high-aspect-ratio nature of the process, with device depths on the order of 10-50 xcexcm and stress-free silicon material, lends itself to the creation of large planar structures several millimeters (mm) in diameter that are ideal for inertial sensing. Large structures are the key to reducing the thermomechanical noise and enabling larger capacitances for improved device sensitivity. The silicon beam structures formed by the process can be tuned for appropriate operation of the gyroscope across wide ranges, and the mass inherent in deep silicon etching improves the resolution achievable by an order of magnitude.
The invention dictates unique electrical connections, crossovers, and actuator mounting methods in order to provide the mechanical decoupling and electrical isolation. The gyroscope is connected to external metal traces and bond pads by means of flexible electrical leads that permit the mechanical member to be encapsulated and protected from the environment. Within the gyroscope itself, one layer of metal is used to route all electrodes, and novel crossover structures are incorporated within the released mechanical element. This capability of arbitrary electrical routing within the gyroscope allows differential measurement for sense capacitors and enhanced efficiency from the capacitive actuators. This segmentation of the design significantly reduces crosstalk between the drive and the sense functions.
The present invention provides design freedom to maintain overall system stability over vibration and temperature by separating the drive and sense frequencies of oscillation. Also, the gyroscope can be operated at modest vacuum using standard sealing materials without the need for expensive getters or vacuum assemblies. The stability presented by the solution is sufficient for packaging in industry standard plastics and operation over automotive-grade temperature ranges.
Because of the reduced electrical and mechanical coupling, the circuit functions required by the gyroscope are easily incorporated into discrete analog components or switched capacitor integrated circuits. The circuit functions are reduced to driving the active member, controlling its amplitude, and detecting the sensing member by means of capacitive electrodes.
Multiple configurations of the gyroscope are possible by rearranging the drive, sense, and input axes to accommodate different configurations. Ultimately, this enables multiple axis configurations within a single package or a full three-degree-of-freedom rate sensing unit.